1. Field of the Invention
The present invention relates to a wiring substrate having a resistor and to a method of manufacturing the same.
2. Description of Related Art
In recent years, a resistor is formed on a main face of a ceramic multilayer wiring substrate by means of sputtering or the like so as to realize impedance matching of signal wiring of the ceramic multilayer wiring substrate. The resistor suppresses reflection of signals, and prevents generation of noise and deterioration of signals.
Meanwhile, a wiring layer (as used herein, “a wiring layer” refers to a stack of conductive layers) is provided on the resistor. This wiring layer includes two or more film-like conductor portions or layers used for, for example, leading signals to the outside via the ceramic multilayer wiring substrate or applying an external voltage via the ceramic multilayer wiring substrate. Also, anther wiring layer (a stack of conductive layers) is provided on the back surface of the ceramic multilayer wiring substrate. This wiring layer includes a film-like conductor portion which is electrically connected to at least one of the above-mentioned two or more film-like conductor portions via the ceramic multilayer wiring substrate and the resistor.
In general, the above-mentioned wiring layer is an electrically conductive film formed of copper, gold, or the like, or a film-like laminate including, for example, a Cu layer, an Ni layer, and an Au layer. In the case of the later film-like laminate, the Ni layer located in the middle functions as a conductive bonding layer for improving the adhesion between the Cu layer located below the Ni layer and the Au layer located above the Ni layer (Patent Document 1).
The above-described wiring layer is formed by a process of uniformly forming a film or a film-like laminate on the resistor and then etching the film or the laminate in the thickness direction. However, if such etching in the thickness direction is performed, fibrous foreign objects, so-called “whiskers,” are formed on the side surface of the film or the Cu layer of the laminate such that the whiskers extend outward. Such “whiskers” cause various problems. For example, adjacent wiring layers formed on the resistor can be electrically connected through the “whiskers” whereby a short circuit is formed between these adjacent wiring layers.
Also, since the above-mentioned etching is generally performed through use of an inorganic acid or an organic acid, etching is performed anisotropically, particularly in the case where adjacent wiring layers are formed on the resistor through etching. Thus, particularly in the case where each of the conductor portions is formed of the above-mentioned film-like laminate, the etching may proceed excessively at a lower portion of the film-like laminate such that an undercut is formed. In such a case, the adhesion strength between the laminate (i.e., the conductor portion) and the resistor decreases, and various problems may occur. For example, the conductor portion may separate from the resistor.